1. Field of the Invention
The present invention relates to a method of recording information on a recording medium using an interaction between light and magnetism.
2. Related Background Art
As a memory using the above-mentioned method, for example, a magnetooptical disk apparatus, or the like, is known. The magnetooptical disk apparatus has been receiving a lot of attention since it has a large storage capacity and is erasable and rewritable. In order to further increase data transfer speed, various studies have been made to achieve an over-write operation, an increase in storage capacity, and the like.
As an over-write method, a so-called magnetic field modulation method is known. In this method, an external magnetic field is modulated in accordance with information to be recorded, while irradiating a light beam of a given power, and the modulated magnetic field is applied to a medium, thereby reversing a direction of magnetization of a recording layer and forming a pit. The magnetic field modulation method is described in detail in U.S. Pat. No. 4,446,004, for example.
However, in the magnetic field modulation method, a formed pit has an arrow-like shape. For this reason, posed are problems of edge variation and erasure error such that a portion to be erased remains. Thus, it is difficult to realize so-called pit length recording by representing information by the length of a pit.
As another over-write method, published European Patent Application 258,978 proposes a method using a medium having two magnetic layers. This method employs a magnetooptical recording medium having a first magnetic layer, and a second layer which has a lower Curie temperature and a higher coercive force at room temperature than those of the first layer, and is exchange-coupled to the first layer. A light beam which is modulated between two power levels (.noteq.0) in accordance with information is irradiated on the medium while applying a given external magnetic field thereto, thereby recording information.
On the other hand, IBM Technical Disclosure Bulletin Vol. 16, No. 17 (December, 1973, pp. 2365 to 2366) discloses a recording method by means of a thermomagnetic transfer process. In this method, an iron-oxide disk constituted by two layers having different coercive forces is used. A track having a wide pattern is recorded on a storage transmission layer having a low coercive force by a magnetic transducer. A portion of the track is scanned by a light beam to transfer only this portion of information on the transmission layer to a main storage layer, thereby accumulating information at a high density. Information is read out by the conventional magnetic transducer in such a manner that information is transferred from the main storage layer to the storage transmission layer.
Japanese Patent Laid-Open (Kokai) No. 63-276731 discloses a method using a disk having a two-layered structure, i.e., a Co-Cr alloy thin film and a Tb-Fe thin film. In this method, information is recorded on the Co-Cr film using a magnetic head, and is transferred to the Tb-Fe film using an optical head. Information is reproduced using a magnetooptical effect such that a light beam is irradiated on the Tb-Fe film and reflected light is detected. With this method, an erasure error can be eliminated, and pit length recording can be easily realized.
However, the method of the thermomagnetic transfer process suffers from the following problems.
1) An apparent coercive force of the low-coercive force layer is increased by an exchange coupling force from the high-coercive force layer, and a considerably strong magnetic field is required to perform recording. For this reason, load on the head is increased, and a high-frequency signal cannot be recorded.
2) In general, a material having a higher Curie temperature exhibits a larger magnetooptical effect. However, since information is reproduced from a layer having a lower Curie temperature, a large reproduction signal is not easily obtained.